In releases of mobile communication standard Long Term Evolution, code division multiplexing of reference signals derived from ZC sequences require reference signals to be based on same base sequences in order to achieve orthogonality. A consequence of this design is that data transmissions on different multiple-input multiple-output (MIMO) layers cannot partly overlap without introducing cross-layer interference when performing channel estimation. In forthcoming 5G (LTE Evolution together with new radio-access technology) systems using massive MIMO and excessive multi-user MIMO, such design would result in either severe pilot contamination or hard scheduling restrictions by not allowing different MU-MIMO users' data transmissions to partly overlap.
Conventional methods that provide low cubic metric and good inter-cell correlation properties do not support partially overlapping users. In the conventional methods, either good cubic metric or good inter-cell correlation properties must be sacrificed in order to support partially overlapping users.
It is desirable to develop methods for scheduling partially overlapping users while maintaining good cubic metric properties of the reference signals.